Ion-exchange membranes made from fluoroorganic polymeric materials having ion-exchange properties are used in a wide variety of applications. Such polymeric materials and membranes may contain fixed anionic groups and associated cations and be capable of exchanging cations, or they may contain fixed cationic groups and associated anions and thus be capable of exchanging anions, or the polymeric materials and the membranes may contain both fixed anionic groups and fixed cationic groups.
Ion-exchange membranes which are essentially hydraulically impermeable but which are permeable to solvated cations or anions, or both, are finding increasing applications in electrochemical devices, for example in fuel cells, in electrolytic cells in which an electrolyte is electrolysed and in electrochemical cells in which electrosynthesis is carried out. In recent years a major development has been in the use of cation-exchange membranes in chlor-alkali cells in which chlorine and an aqueous metal hydroxide are produced by the electrolysis of aqueous alkali metal chloride solution.
Although many fluoroorganic polymeric materials have been proposed for use as membranes for such chlor-alkali cells, in recent years perfluoroorganic polymers containing ion-exchange groups, particularly fixed sulphonic and carboxylic groups, have found favour in chlor-alkali cells on account of the resistance of such perfluoroorganic polymers to chemical degradation in the cell.
As examples of such perfluoroorganic polymers containing ion-exchange groups may be mentioned inter alia the perfluoroorganic polymeric materials containing sulphonic groups described in GB patent 1034197 and the perfluoroorganic polymeric materials containing carboxylic groups described in GB patents 1516048 and 1518387.
Fluoroorganic polymeric materials comprising a copolymer derived from a substantially fluorinated alkene and a perfluorocarbon vinyl ether bearing an ion-exchange group or group convertible thereto are known, for example copolymers of tetrafluoroethylene (TFE) and a perfluorocarbon vinyl ether sulphonyl fluoride, eg CF.sub.2 .dbd.CFOCF.sub.2 C(CF.sub.3)FOCF.sub.2 CF.sub.2 SO.sub.2 F wherein the moiety OCF.sub.2 C(CF.sub.3)FOCF.sub.2 CF.sub.2 SO.sub.2 F forms a pendent group from the polymer backbone. Such copolymers, after appropriate hydrolysis of the sulphonyl group in the pendent group to the free sulphonic acid, have been suggested for use as inter alia ion-exchange membranes for use in, for example, diffusion dialysis, electrolytic cells and fuel cells.
Recently, it has been shown in U.S. Pat. No. 4,358,545 that an improved membrane for use as an ion-exchange membrane in a chlor-alkali cell can be obtained by using a fluoroorganic polymeric material comprising a copolymer as described above wherein the pendent group is relatively short, eg in a copolymer derived from TFE and CF.sub.2 .dbd.CFOCF.sub.2 CF.sub.2 SO.sub.2 F. It has been suggested that this improvement is the result of less water being absorbed per sulphonic group at a given concentration of functional group and allows the use of a polymer of lower equivalent weight (EW) in chlor-alkali cells.
More recently, U.S. Pat. No. 4,940,525 has confirmed the advantages of such copolymers with relatively short pendent groups and furthermore suggested that for certain applications, for example in fuel cells, eg so-called solid polymer electrolyte fuel cells (SPFC), such copolymers should have a low EW and a certain maximum Hydration Product.
"Hydration Product" is defined in U.S. Pat. No. 4,358,545 as the mathematical product of the hydration of the copolymer, expressed as moles of adsorbed water per functional group, and the EW weight of the copolymer.
In our EP 0,331,321 it has been shown that the ion-exchange group in a fluoroorganic polymeric material may be joined to the polymer backbone through a saturated cyclic group and that by suitable positioning of the ion-exchange group on the saturated cyclic group, and introduction of other substituents which are not ion-exchange groups onto the saturated cyclic group, the ion-exchange activity of the ion-exchange group may be varied. It has been shown, in particular, that it is possible to achieve a relatively high ion-exchange capacity in a fluoroorganic polymeric material which contains a relatively low proportion of units derived from the perfluorocarbon vinyl ether monomer disclosed therein.
In our EP 0,345,964 it has been shown that membranes prepared from homogeneous blends of organic polymeric material comprising a fluoropolymer containing ion-exchange groups and a fluoropolymer which is substantially free of ion-exchange groups, in defined ratio, are particularly effective in preventing hydraulic transfer of electrolyte between the anode compartments and the cathode compartments of an electrolytic cell in which the membrane is installed.
By homogeneous blend therein there is meant an intimate mixture such as may be formed, for example by melt-blending the fluoropolymers, that is by mixing the fluoropolymers at or above a temperature at which the fluoropolymers are in a fluid/molten state; or by blending the fluoropolymers in a solvent in which both fluoropolymers are soluble, and thereafter separating the blend of fluoropolymers from the solution; or by blending the polymers in the form of an emulsion of the polymers in a liquid medium and separating the homogeneous blend by coagulation. A homogeneous blend is not formed merely by forming a mixture of the solid polymers in particulate form.